I can't say for certain but I think it has something to do with the old HTML font tag. It supported a "size" attribute between 1 and 7. 3 was the default so I t

I can't say for certain but I think it has something to do with the old HTML font tag. It supported a "size" attribute between 1 and 7. 3 was the default so I think H5 and H6 might have corresponded to font tag sizes 1 and 2.